The long range goal of this project is the elucidation of the mechanism whereby light causes the initial neural response in the retina. It is proposed that studies be made on the visual pigment, rhodopsin, as well as the rod receptor potential. Since spectral changes in the pigment do not seem to correlate with the production of the receptor potential, it is proposed that changes in the pigment protein, opsin, be studied by means other than conventional spectrophotometry. Work on this project has already described certain "isochromic" changes in opsin and work in the future is aimed at a more complete description of such changes. When the changes have been characterized, the correlation with the receptor potential may become evident. This requires that, simultaneously, work must also proceed on the receptor potential. The correlations that will be sought include temporal, pharmacologic and energetic characteristics.